Optical submarine cable systems are the basis of global networks. Thanks to advances in higher bit rate modulation technology and wavelength multiplexing technology, some optical submarine cable systems achieve traffic capacities greater than 1 terabit per second.
FIG. 5 is a diagram illustrating a conventional configuration of an optical submarine cable system related to the present invention. An optical submarine cable system 900 includes submarine cables 901 containing protected optical fibers, optical submarine repeaters 902 which compensate for attenuation along the optical fibers, optical branching units 903 each of which splits a submarine cable into two, and landing stations (stations A to F) 904 which are installed on land. The optical branching units 903 branch or couple wavelength division multiplexing (WDM) signals input from adjacent landing stations A to F or optical branching units 903. The optical branching units 903 are classified to a “fixed type” which branches or couples pairs of optical fibers while regarding two optical fibers as one pair and an “optical add-drop type” which branches optical signals of different wavelengths in one fiber pair.
Patent Literature 1 discloses a ring network system and a node unit that are used in an optical submarine cable system and include through, add, and drop functions.